The invention concerns heating of hydrocarbon materials in geological subsurface formations by radio frequency electromagnetic waves (RF), and more particularly, this invention provides a method and apparatus for heating hydrocarbon materials in geological formations by RF energy emitted by well casings that are coupled to an RF energy source.
Hydrocarbon materials that are too thick to flow for extraction from geologic deposits are often referred to as heavy oil, extra heavy oil and bitumen. These materials include oil sands deposits, shale deposits and carbonate deposits. Many of these deposits are typically found as naturally occurring mixtures of sand or clay and dense and viscous petroleum. Recently, due to depletion of the world's oil reserves, higher oil prices, and increases in demand, efforts have been made to extract and refine these types of petroleum ore as an alternative petroleum source.
Because of the high viscosity of heavy oil, extra heavy oil and bitumen, however, the drilling and refinement methods used in extracting standard crude oil are frequently not effective. Therefore, heavy oil, extra heavy oil and bitumen are typically extracted by strip mining of deposits that are near the surface. For deeper deposits wells must be used for extraction. In such wells, the deposits are heated so that hydrocarbon materials will flow for separation from other geologic materials and for extraction through the well. Alternatively, solvents are combined with hydrocarbon deposits so that the mixture can be pumped from the well. Heating with steam and use of solvents introduces material that must be subsequently removed from the extracted material thereby complicating and increasing the cost of extraction of hydrocarbons. In many regions there may be insufficient water resources to make the steam and steam heated wells can be impractical in permafrost due to unwanted melting of the frozen overburden. Hydrocarbon ores may have poor thermal conductivity so initiating the underground convection of steam may be difficult to accomplish.
Another known method of heating thick hydrocarbon material deposits around wells is heating by RF energy. Prior systems for heating subsurface heavy oil bearing formations by RF have generally relied on specially constructed and complex RF emitting structures that are positioned within a well. Prior RF heating of subsurface formations has typically been vertical dipole antennas that require specially constructed wells to transmit RF energy to the location at which that energy is emitted to surrounding hydrocarbon deposits. U.S. Pat. Nos. 4,140,179 and 4,508,168 disclose such prior dipole antennas positioned within vertical wells in subsurface deposits to heat those deposits. Arrays of dipole antennas have been used to heat subsurface formations. U.S. Pat. No. 4,196,329 discloses an array of dipole antennas that are driven out of phase to heat a subsurface formation. Prior systems for heating subsurface heavy oil bearing formations by RF energy have generally relied on specially constructed and complex RF emitting structures that are positioned within a well.